The principle of concentrating solar power to obtain a source of heat can probably be ascribed to Archimedes. It is only in modern times however, that concentration solar power plants have become viable. In particular the most reliable technology makes use of parabolic troughs as thermal solar concentrators. The solar field has a modular structure, made of linear parabolic collectors, tied up in series, and set along parallel lines several hundreds of meters long. Each collector is made of a reflector of parabolic shape which concentrates the sun rays on a receiving pipe, placed along on the focal line. A thermo-vector fluid pumped up through the receiving pipes, supplies heat to a power station placed at the centre of the solar field. The heat is used to produce steam that sets in action an electrical turbo-generator group.
The first plants of this kind were built in the Mojave Desert at the beginning of the 80s. In Europe the largest plants, the Andasols, are currently built in Spain according to the same basic scheme. All these plants use an inflammable and toxic mineral oil, as thermal working fluid. This diathermic oil is limited in particular to a maximum temperature of about 400° C. Above this threshold, there is a substantial risk of igniting a burning reaction. This means that the efficiency of the plants is necessarily rather low.
In Italy, in the Enea laboratories, the Archimede project has been developed to overcome the diathermic oil limitation. In this case the working fluid is a mixture of molten salts commonly used as fertilizers, namely 60% NaNO3 and 40% KNO3. They can be safely used up to about 550° C., but still have the substantial problem that below about 200° C. they solidify. This means that the main circuit must be kept operational also overnight, using the stored heat or, eventually, heating it with an outside source. Moreover, since efficient steam turbines operate at temperatures of at least 500° C., the stored efficient heat is limited.
Two important components of such solar concentration plants are the receiver pipe (in the solar collector) and the heat storage.
The most advanced receiver pipe on the market today is the so called Angeloantoni one, that has also been developed within the frame of the Archimede project. It is constituted by a metallic pipe, blackened through a super black technique, that is inserted into a glass pipe. Between the glass pipe and the metallic one, vacuum is made in order to create an efficient thermal insulation. Overall this pipe is extremely efficient but fragile, since the two concentric pipes are made of different materials, glass and metal, with different elongation coefficients. This limitation is not particularly severe if the circuit is kept always operational with the working fluid flowing continuously at high temperature, but it is not acceptable in any situation where the working fluid would be halted to cool.
The heat storage, instead, allows to produce steam also in absence of solar radiation for a prolonged period of time. There are cases, such as the Andasol plants, where heat is stored in molten salts reservoirs. The Archimede project does the same. A more innovative project is that of the Australian Cloncurry power plant, a tower plant, that is a totally different approach with respect to trough plants, that stores heat on the top of the tower into purified graphite blocks. Nobody has however still tried to store heat for periods of time longer than a few weeks since heat dispersion is a serious limitation.
Patents or patents applications regarding concentrating solar power plants are also known.
WO2009/004476 discloses a modular assembly for the production and accumulation of solar energy with the fused salts technology. The assembly comprises a tank for the accumulation of fused salts, a reflecting body for capturing the solar rays, a tubular collector, a parabolic body, a delivery circuit and a return circuit.
The tubular collector faces the parabolic body and the delivery and return circuit communicate on one side with the tank and on the other with the collector for supplying the fused salts through the collector. The accumulation tank is set in a position underlying the collector and the delivery and returns circuits are set so as to enable unloading by gravity of the fused salts into the accumulation tank.
WO79/01004 describes a solar energy system comprising a collector, a receiver, a thermal energy accumulator and a thermal energy transfer system. The collector is used to collect and concentrate solar radiation, the receiver associated with the collector is employed for converting the radiation concentrated by the collector into thermal energy, and the thermal energy transfer system is used to transfer thermal energy from the receiver to the thermal energy accumulator. The thermal energy accumulator comprises a mixture of fusible salts.